Hard alloy functionally graded material molding method

ABSTRACT

A hard alloy functionally graded material molding method, comprising: firstly, mixing an additive with an alloy raw material to obtain a mixture; then placing the mixture obtained from the above step into a composite die set for composite pressure molding, so as to obtain a blank; the composite die set comprise outer layer high-expansion coefficient dies, an intermediate transition layer die set, and inner layer low-expansion coefficient dies; and finally, sintering the above blank to obtain the hard alloy functionally graded composite material. The multi-component hard alloy functionally graded composite material which is large in size, has a complex outline structure and is free from obvious interfaces is prepared. In a thickness direction, the sintering molded functionally graded material attains gradient grain structures that have different components and different grain sizes and are free from obvious interfaces.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority to a Chinese Patent Application No. 201510305122.6, filed on Jun. 5, 2015, and titled with “HARD ALLOY FUNCTIONALLY GRADED MATERIAL MOLDING METHOD”, and the disclosures of which are hereby incorporated by reference.

FIELD

The present invention relates to the field of functionally graded material technology, more specifically, relates to a molding method for hard alloy functionally graded material.

BACKGROUND

Composite material is a material comprising two or more materials of different properties, which form a new material with new properties on macroscopic (microscopic) scale by physical or chemical methods. Materials with different properties complement each other and result in a synergistic effect, giving a better comprehensive property than the initial component materials so that the composite material will satisfy different requirements.

Functionally graded material is a relatively special type of composite material among multitudinous composite materials. Unlike normal composite materials, functionally graded material is made from two (or more) materials with different properties. By adjusting the components and structures of the two (or more) materials gradually, the interfaces disappear and the properties of material change slowly accompany with the changing of the components and the structures of material. Functionally graded materials have a character of gradually changed components in a certain space direction, thus, they can overcome the disadvantage of conventional composite materials efficiently. Functionally graded materials can be used as interface layer to connect two incompatible materials, improving the bonding strength greatly. Besides, the functionally graded material can also be used as interface layer to decrease the residual stress and thermal stress, and eliminate the singularity of interface junctions and stress free endpoints. In addition, replacing conventional homogenous materials to functionally graded material improves the connection strength and decreases the crack driving force at the same time.

With the rapid development of global economy, many industries, for example, petroleum industry, chemical industry, energy industry, electric sector, metallurgical industry, space industry and so on, require a large amount of mechanical parts that have to bear wear and tear in conditions like high-temperature oxidation and corrosion. It requires the material not only with well abrasion resistance, erosion resistance and oxidation resistance, but also excellent strength-toughness. However, homogenous materials with homogenous properties often cannot satisfy the requirements of the applications referred above.

Therefore, how to prepare a functionally graded material with excellent abrasion resistance, good strength-toughness, strong corrosion resistance and high-temperature oxidation resistance is one of the major subjects of composite materials research in long term.

SUMMARY

In view of the above, the technical problem to be solved by the present disclosure is to provide a molding method for functionally graded material which has excellent abrasion resistance, good strength-toughness, strong corrosion resistance and oxidation resistance at high temperature, especially refers to a molding method for a hard alloy functionally graded material.

In order to solve the technical problems above, the present disclosure provides a hard alloy functionally graded material molding method, comprising:

A) mixing an additive with an alloy raw material to obtain mixture;

B) placing the mixture obtained from the above step into a composite die set for composite pressure molding to obtain a blank;

the composite die set comprises an outer layer die with high expansion coefficient, an intermediate transition layer die set, and an inner layer die with low expansion coefficient; and

C) sintering the above blank to obtain a hard alloy functionally graded composite material.

Preferably, in the mixture mentioned above, the mass percentage of alloy raw materials is from 50% to 85% and the mass percentage of the additive is from 15% to 50%.

Preferably, step A comprises:

mixing an additive and a first alloy raw material together to prepare a mixture for surface layer; then, mixing an additive and a second raw material together to prepare a mixture for intermediate layer; finally, mixing an additive and a third raw material to prepare a mixture for inner layer.

Preferably, the Fisher particle size of the mixture for surface layer is less than or equal to 3 μm, and the Fisher particle size of the mixture for intermediate layer is from 0.5 to 5 μm, and the Fisher particle size of the mixture for inner layer is from 3 to 30 μm.

Preferably, the alloy raw material above comprises a hard phase and a soft phase;

the hard phase is tungsten carbide, and the soft phase is cobalt, iron or nickel.

Preferably, the mass content of the hard phase in the first alloy raw material is 93% to 97% of the mass of the first alloy raw material; and the mass content of the hard phase in the second alloy raw material is 84% to 95% of the mass of the second alloy raw material; and the mass content of the hard phase in the third alloy raw material is 75% to 90% of the mass of the third alloy raw material.

Preferably, the composite pressure molding is selected from warm compaction, injection molding and hot isostatic pressing molding, or a mixture thereof.

Preferably, the additive is selected from polyethylene, paraffin, polyethylene glycol, polypropylene, polystyrene, stearic acid, dimethyl phthalic acid, dibutyl phthalic acid and EVA, or a mixture thereof.

Preferably, the blank is subjected to degreasing treatment before sintering; the degreasing treatment is thermal degreasing and/or solvent degreasing.

Preferably, the sintering is vacuum pressure sintering or hot isostatic pressing sintering.

The present disclosure provides a molding method for hard alloy functionally graded material, comprising: mixing additive with alloy raw material to obtain mixture; placing the mixture obtained from the above step into a composite die set for composite pressure molding to obtain a blank; the composite die set comprises an outer layer die with high expansion coefficient, an intermediate transition layer die set, and an inner layer die with low expansion coefficient; finally, sintering the blank to obtain the hard alloy functionally graded composite material. Compared with conventional technology, the design concept of preparing functionally graded material is used in preparing a hard alloy, giving the hard alloy excellent strength-toughness while maintaining its excellent abrasion resistance, corrosion resistance and high-temperature resistance at the same time. In addition, powder metallurgy method and composite die set molding are used to prepare the hard alloy functionally graded material in the present disclosure. By adjusting the particle size and composition of the metal powder, additive ratio, different kinds of pressure molding methods and technological parameters, composite die set shape and sintering technology, in considerably large thickness range, the complex outline structured and composition continuous adjustable hard alloy functionally graded material with high abrasion resistance and excellent strength-toughness can be easily prepared. According to the results of experiment, the hard alloy functionally graded material prepared in present disclosure has a lateral breaking strength of 3310 N/mm², coercivity of 9.3 kA/m, and a percentage of magnetic cobalt (Com %) of 9.19. In addition, from the surface to the core, the hard alloy functionally graded material not only has compact structure and gradient transitional components and hard particle size, but also has no defect such as pore, bubble and crack.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is the metallography image of the powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 100× magnification;

FIG. 2 is the metallography image of the surface layer of the powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 3 is the metallography image of the intermediate transitional layer G1 of the powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 4 is the metallography image of the intermediate transitional layer G2 of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 5 is the metallography image of the intermediate transitional layer G3 of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 6 is the metallography image of the intermediate transitional layer G4 of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 7 is the metallography image of the core of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 8 is the structure representation of the WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure.

DETAILED DESCRIPTION

The technical solution in the examples of the present disclosure will be described clearly and completely herein. Apparently, the described examples are only a part of the examples of the present disclosure, rather than all examples. Based on the examples in the present disclosure, all of other examples, made by one of ordinary skill in the art without any creative efforts, fall into the protection scope of the present disclosure.

The present disclosure provides a hard alloy functionally graded material molding method, comprising:

-   -   A) mixing an additive with an alloy raw material to obtain a         mixture;     -   B) placing the mixture obtained from the above step into a         composite die set for composite pressure molding to obtain a         blank;         -   the composite die set comprises an outer layer die with high             expansion coefficient, an intermediate transition layer die             set and an inner layer die with low expansion coefficient;             and     -   C) sintering the blank to obtain a hard alloy functionally         graded composite material.

In the present disclosure, additive and alloy raw materials are mixed first to give the mixture. The additive is preferably selected from polyethylene, paraffin, polyethylene glycol, polypropylene, polystyrene, stearic acid, dimethyl phthalic acid, dibutyl phthalic acid and EVA, or a mixture thereof; more preferably polyethylene, paraffin, polyethylene glycol, polypropylene, polystyrene, stearic acid, dimethyl phthalic acid, dibutyl phthalic acid or EVA; most preferably polyethylene, paraffin, polyethylene glycol and stearic acid, or a mixture thereof. There is no special restriction on the alloy raw material, and the alloy raw materials can be any of the well-known hard alloy raw materials to those of ordinary skill in the art. Preferably, the alloy raw material used in the present disclosure comprises a hard phase and a soft phase. The hard phase is preferably tungsten carbide. The soft phase, i.e., binding phase, is preferably selected from cobalt, iron, molybdenum and nickel, or a mixture thereof; more preferably cobalt, iron or nickel; most preferably cobalt. The preferred alloy raw material further comprises the element that can increase the properties of the hard alloy, which is preferably selected from carbon, boron, tungsten, molybdenum, chromium, vanadium, tantalum, titanium, nickel, iron and carbide thereof, or a mixture thereof, more preferably tantalum carbide or titanium carbide. In the mixture, the mass percentage of alloy raw material in the mixture is preferably from 50% to 85%, more preferably from 55% to 80%, and most preferably from 60% to 75%. The mass percentage of the additive in the mixture is preferably from 15% to 50%, more preferably from 25% to 45%, and most preferably from 30% to 40%.

There is no special restriction on the detailed mixing step, which can be any mixing process familiar to the person skilled in this art. In order to ensure the effect of composite pressure molding and sintering, preferred steps are as follows:

mixing additive with the first alloy raw material to prepare mixture for surface layer; mixing additive with the second raw material to prepare mixture for intermediate layer; mixing additive with the third raw material to prepare mixture for inner layer.

In the present disclosure, additive is mixed with the first alloy raw material to prepare the mixture for surface layer, which is a fine size alloy particle powder. The Fisher particle size of the mixture for surface layer is preferably less than or equal to 3 μm, more preferably from 0.5 μm to 3 μm, even more preferably less than or equal to 2.5 μm, and most preferably from 0.5 μm to 2 μm. The mass percentage of the alloy hard phase in the mixture for the first alloy raw material is preferably from 93% to 97%, more preferably from 93.5% to 96.5%, even more preferably from 94% to 96%, and most preferably from 94.8% to 95.2%; the percentage, that is, the percentage of the particle content of alloy hard phase in total mass of the alloy particle powder. The percentage of the additive in the mixture for the surface layer is preferably from 40% to 50%, more preferably from 41% to 49%, and most preferably from 43% to 47%.

There is no special restriction on the mixing method, which can be any of the hard alloy mixing method well-known to one of ordinary skill in the art, and the ball-milling is preferred in the present disclosure. There is no special restriction on mixing equipment, which can be any of the hard alloy mixing equipment well-known to one of ordinary skill in the art. There is no special restriction on other conditions for the mixing process, which can be any of the hard alloy mixing conditions well-known to the skilled in the art.

In the present disclosure, the surface layer mixture with particular Fisher particle size is obtained by the mixing process foregoing and subjected to pressure molding process and sintering to give a fine grained material. The fine grained material with particular particle size and composition are used in the surface layer of the product. By using the material with high content hard particle and fine grains, the hard alloy functionally graded material with excellent surface abrasion resistance is obtained.

In the present disclosure, additive is mixed with the second alloy raw material to prepare the mixture for the intermediate layers, which is an alloy powder with fine size and medium size particles. The Fisher particle size of the mixture for the intermediate layer is preferably from 0.5 μm to 5 μm. In the second raw material, the mass percentages of the alloy hard phase in the mixtures for the second alloy raw material are preferably from 84% to 95%; the percentage, that is, the percentage of the particle content of hard phase in total mass of the alloy particle powder. The mass percentage of the additive in the mixture for intermediate layer is preferably from 30% to 45%, more preferably from 35% to 44%, and most preferably from 40% to 43%. There is no special restriction on other components in the mixture for the intermediate layer, and those of ordinary skill in the art can choose any other well-known raw material according to the demands of production or requirements for quality. In the present disclosure, TaC is added into the mixture for the intermediate layer (the intermediate transition layer), and the mass percentage of the TaC is preferably from 0.2% to 0.4%, more preferably from 0.25% to 0.35%, and most preferably 0.3%.

There is no special restriction on the mixing method, which can be any of the hard alloy mixing method well-known to one of ordinary skill in the art, and the ball-milling is preferred in the present disclosure. There is no special restriction on mixing equipment, which can be any of the hard alloy mixing equipment well-known to one of ordinary skill in the art. There is no special restriction on other conditions for the mixing process, which can be any of the hard alloy mixing conditions well-known to the skilled in the art.

In the present disclosure, the mixture for intermediate transition layer with particular Fisher particle size is obtained by the mixing process foregoing and then the multi-sized alloy powder with fine size and medium size is subjected to pressure molding process and sintering to give a material with duplex grained structure which has a size between the fine size of surface layer and medium size of the inner layer. The material with medium size grains structure is used in the intermediate transition layers of the products. By controlling the components and the size of grains, the gradually transitional alloy functionally graded material with no obvious interface can be prepared, ensuring the excellent comprehensive mechanical properties of the product. In addition, a certain amount of TaC is preferably added to inhibit the development of the WC fine particles during the sintering process in the present disclosure.

In the present disclosure, additive is mixed with the third alloy raw material to obtain the mixture for the inner layer, which is a mixture of the huge size and medium size particle alloy powder. The Fisher particle size of the mixture for the inner layer is preferably from 3 μm to 30 μm, more preferably from 5 μm to 25 μm, even more preferably from 7 μm to 20 μm, and most preferably from 8 μm to 15 μm. In the third raw material, the mass percentage of the alloy hard phase in the mixture for the third alloy raw material is preferably from 75% to 90%, more preferably from 77% to 88%, even more preferably from 80% to 85%, and most preferably from 82% to 84%; the percentage is the percentage of the particle content of the alloy hard phase in total mass of the alloy particle powder. The volume ratio of the additive in the mixture for inner layer is preferably from 15% to 45%, more preferably from 25% to 42%, and most preferably from 35% to 40%.

There is no special restriction on the mixing method, which can be any of the hard alloy mixing method well-known to one of ordinary skill in the art, and ball-milling is preferred in the present disclosure. There is no special restriction on mixing equipment, which can be any of the hard alloy mixing equipment well-known to one of ordinary skill in the art. There is no special restriction on other conditions for the mixing process, which can be any of the hard alloy mixing conditions well-known to the skilled in the art.

In the present disclosure, the mixture for the inner layer with a certain Fisher particle size is obtained by the foregoing mixing process. The alloy powder with huge size and medium size is subjected to pressure molding process and sintering to give a coarse-grained material with certain particle size and components, which is used in the inner layer of the products, in order to satisfy the properties requirements like long duration and high toughness of the functionally graded material.

In the present disclosure, the mixtures for the surface layer, the intermediate layer and the inner layer are obtained after the mixing processes. The mixtures obtained from the above step are placed into composite dies respectively for pressure molding to obtain the blanks. The composite die set preferably comprises an outer layer die with high expansion coefficient, an intermediate transition layer die set, and an inner layer die with low expansion coefficient. There is no special restriction on the number of different die sets, and the skilled person in the art can adjust the number of the die sets according to the actual production conditions and product requirements. For example, the die set for intermediate layer can be only one or more accordingly. There is no special restriction on the composite pressure molding, which can be any of the composite pressure molding methods of functionally graded materials well-known to the skilled person in the art. In the present disclosure, the composite pressure molding is preferably selected from the group comprising of warm compaction, injection molding and hot isostatic pressing molding, more preferably warm compaction, injection molding or hot isostatic pressing molding. There is no special restriction on the condition for composite pressure molding in the present disclosure, which can be any of the well-known composite pressure molding conditions of functionally graded material to the person skilled in the art. And the molding temperature is preferably 130° C. to 145° C., more preferably 135° C. to 140° C.; the molding pressure is preferably 2 MPa to 5 MPa, more preferably 3 MPa to 4 MPa.

The conception of the above preparation steps in the present disclosure is stated below. According to the requirement of the desired molding part, the first alloy raw material is used in the surface, i.e., the first hard alloy material (Y1); the third hard alloy material (Y2) is used in the core (inner layer) of the part. The mixture for the surface layer (Y1+T1) can be obtained by mixing the first hard alloy material and a certain amount of additive together, wherein the first hard alloy material has the first coefficient of thermal expansion (al). In addition, the mixture for the inner layer (Y2+T2) can be obtained by mixing the third hard alloy material and a certain amount of additive together, wherein the third hard alloy material has the second coefficient of thermal expansion (α2). The first coefficient of thermal expansion is different from the second coefficient of the thermal expansion, and it is preferred that the first coefficient of thermal expansion is slightly larger than the second coefficient of thermal expansion. Moreover, by adding a certain amount of additive (Tn), one or more types of mixture for the intermediate layer is made in the present disclosure. As a consequence, the blank of intermediate layer, i.e., intermediate gradient region (Y1*n %+Y2*(100−n) %+Tn), is a transitional layer between the first alloy material and the third alloy material (Y1 and Y2), forming a gradually transitional intermediate composite gradient region between the surface layer and the inner layer, which has a coefficient of thermal expansion (an) between α1 and α2, and the transition is homogeneous.

By design of the steps above, the present disclosure realizes the transition with no interfaces from the fine particle hard alloy component of the surface layer to coarse particle tough alloy component of the inner layer, improving the abrasion resistance and strength of the molded component part and decreasing the crack driving force simultaneously.

In the present disclosure, the blank prepared by above steps is sintered to obtain the hard alloy functionally graded material. There is no special restriction on the sintering method, which can be any of the well-known sintering methods to those of ordinary skill in the art. Preferably, vacuum and pressure sintering or hot isostatic pressing sintering is used in the present disclosure, more preferably vacuum and pressure sintering. The sintering temperature of the vacuum and pressure sintering is preferably 1380° C. to 1420° C., more preferably 1390° C. to 1410° C., and most preferably 1395° C. to 1405° C. The protective gas for vacuum and pressure sintering is preferably selected from nitrogen, hydrogen and noble gas, or a mixture thereof. There is no special restriction on the sintering equipment, which can be any of the sintering equipment well-known to those of ordinary skill in the art. There is no special restriction on other conditions of sintering process, which can be any of the relevant sintering condition well-known to those of ordinary skill in the art.

There are may be other steps before and after the sintering, which is not limited in the present disclosure, and the person skilled in the art can adjust these steps according to the actual conditions. In order to improve the quality and performance of final functionally graded material, in addition to the volatile additive with low melting point, degreasing treatment is preferably performed before sintering. The degreasing treatment is preferably thermal degreasing or solvent degreasing. The thermal degreasing is processed as follows: heating and insulating the blanks at the temperature below 500° C. in a protective atmosphere of nitrogen, hydrogen, or inert gas to remove the additives in the blanks absolutely. The solvent degreasing, i.e., soak degreasing, is to partly remove the additives in the blanks by soaking the blanks into organic solvents; preferably, the blanks are put into gasoline between 55° C. and 90° C. for 25 h to 60 h. There is no special restriction on other conditions for the two types of degreasing treatments, which can be any relevant condition well-known to the skilled person in the art.

By the steps above, the hard alloy functionally graded composite material having multi-component and free of obvious interface is prepared in one effort, which is in large size and has a complex outline structure. In thickness direction, the molded functionally graded material after sintering has gradient grain structures that have different components, grain sizes and no obvious interfaces, giving the molded hard alloy material excellent comprehensive mechanical properties integrating high hardness, abrasion resistance, strength and toughness. The component parts prepared by the method of the present disclosure can be widely used in industries which require a large amount of mechanical parts that have to bear wear and tear in conditions like high-temperature oxidation and corrosion, for example, petroleum industry, chemical industry, energy industry, electric sector, metallurgical industry, space industry and so on.

The process control test demonstrates that, in the present disclosure, the blank after removing the additives has a gradient decreasing distribution of porosity from the surface layer to the core. In sintering process, the axial sintering shrinkage and radial sintering shrinkage of surface layer, core (inner layer), and intermediate transition region (intermediate layers) are almost the same, with a fluctuation lower than or equal to 0.5%.

The experiment results show that the hard alloy functionally graded material prepared in the present disclosure has a transverse rupture strength of 3310 N/mm², coercivity of 9.3 kA/m, and the percentage of magnetic cobalt in hard alloy (Com %) is 9.19. From the surface to the core, the material has compact structure and gradient transition of components and hard particle size, no defect such as pore, bubble and crack. The results above indicate that the present disclosure realize the transition without interfaces from fine particle hard alloy component of the surface to coarse particle tough-tough alloy component of the core, improving the abrasion resistance and strength of the molded part and simultaneously decreasing its crack driving force.

In order to further illustrate the technical solution of the present disclosure, the preferred embodiment of the present disclosure is described hereinafter in conjunction with the examples of the present disclosure. It is to be understood that the description is merely illustrating the characters and advantages of the present disclosure, and is not intended to limit the claims of the present application.

Example 1

Preparation of WC—Co Hard Alloy Functionally Graded Composite Material

The first alloy raw material was prepared by mixing WC particles and Co powder together, wherein the mass percentage of WC particles is 95% and Co powder is 5%. The additive for the surface layer mixture was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 18%, 57%, 23%, and 2%, respectively. The additive and the first alloy raw material was mixed at a ratio of 44:56 and subjected to ball-milling treatment to give a mixture for the surface layer with a Fisher particle size of 0.5 μm to 1.2 μm.

The second alloy raw material was prepared by mixing WC particles, TaC particles and Co powder together, wherein the mass percentage of WC particles is from 83.7% to 94.7%, TaC particle 0.3%, and Co powder as balance. The additive for the mixture for intermediate layer was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 18% to 20%, 55% to 57%, 22% to 23%, and 2% to 3%, respectively. The additive and the second alloy raw material were mixed at a ratio of 42:58 and subjected to ball-milling treatment to give four groups of mixture with a Fisher particle size of 0.5 μm to 5 μm for the intermediate layer.

The third alloy raw material was prepared by mixing WC particles and Co powder together, wherein the mass percentage of WC particles is 85% and Co powder 15%. The additive for the inner layer mixture was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 20%, 55%, 22%, and 3%, respectively. The additive and the third alloy raw material were mixed together at a ratio of 40:60 and subjected to ball-milling treatment to give a mixture for the inner layer with a Fisher particle size of 5 μm to 9 μm.

According to the outline structure of the component part, the expansion coefficient of surface layer mixture, inner layer mixture and intermediate transition layer mixture, an outer layer die with high-expansion coefficient, four intermediate transition layer dies and an inner layer die with low-expansion coefficient, were designed.

The mixture for the surface layer, the mixtures for the intermediate layers, and the mixture for the inner layer were put into the six composite dies in order and subjected to warm compaction molding process at 135° C. and 4 MPa to give a blank.

Under the protection of argon gas, the blank was heated to 200° C. and insulated for 1 h; then it was reheated to 250° C. and insulated for 3 h; further, it was reheated to 450° C. and insulated for 2.5 h; finally, it was cooled by nature cooling to room temperature to remove the additive in the blank completely.

The blank after the thermal degreasing treatment was put into vacuum pressure furnace and pressure sintering was carried out at 1400° C. with the protection of nitrogen gas to give a hard alloy functionally graded material after insulation cooling.

The process control test shows that the blank after removing the additives has a gradient decreasing distribution of porosity from the surface layer to the core. After sintering, the sintering shrinkage of the six blanks with different particle sizes and components from the surface to the core ranges from 1.237 to 1.243, which indicates that during the sintering treatment, the axial sintering shrinkage and radial sintering shrinkage of surface layer, core (inner layer), and intermediate transition region (intermediate layer) are almost the same, with a fluctuation lower than or equal to 0.5%.

The composition distribution of the hard alloy functionally graded material prepared in Example 1 was investigated, and the results are shown in Table 1. Table 1 shows the composition distribution of the WC—Co hard alloy functionally graded composite material prepared in Example 1.

TABLE 1 the composition distribution of the WC-Co hard alloy functionally graded composite material prepared in Example 1 Distance Mass Ratio Mass Ratio to Surface of WC of Co Layer Layer (mm) (%) (%) Surface Layer (a)   0-0.4 95 5 Intermediate G1 0.4-0.7 92.7 7 Layer G2 0.7-0.9 90.7 9 G3 0.9-1.1 88.7 11 G4 1.1-1.4 86.7 13 Inner Layer (b) 1.4-core 85 15

The hardness distributions of the hard alloy functionally graded material prepared in Example 1 was investigated, and the results are shown in Table 2. Table 2 shows the hardness distribution along thickness direction of the molding part made from the WC—Co hard alloy functionally graded composite material prepared in Example 1.

TABLE 2 The hardness distribution along thickness direction of the molding part made from WC—Co hard alloy functionally graded composite material prepared in Example 1 Distance to Surface Layer (mm) 0.2 0.5 0.8 1.0 1.2 1.8 HV 1595 1438 1351 1310 1267 1190 Converted 91.6 90.4 89.7 89.2 88.6 88.0 into HRA

The properties of the hard alloy functionally graded material prepared in Example 1 were investigated. The results show that, the metal-based functionally graded composite material prepared in Example 1 has a density of 14.41 g/cm³, transverse rupture strength of 3460 N/mm², coercivity of 9.86 kA/m and magnetic cobalt percentage (Com %) of 9.82. The results demonstrate that the hard alloy functionally graded material prepared by the method of the present disclosure has excellent performances.

The hard alloy functionally graded material prepared in Example 1 was subjected to metallographic analysis.

Reference is made to FIG. 1, which is the metallography image of the powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 100× magnification.

As shown in FIG. 1, under microscope at 100× magnification, the porosity of hard alloy functionally graded material prepared in Example 1 is A02, B00.

FIG. 2 is the metallography image of the surface layer of Powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 3 is the metallography image of the intermediate transitional layer named G1 of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 4 is the metallography image of the intermediate transitional layer named G2 of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 5 is the metallography image of the intermediate transitional layer named G3 of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification;

FIG. 6 is the metallography image of the intermediate transitional layer named G4 of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification; and

FIG. 7 is the metallography image of the core of powder metallurgy product of WC—Co hard alloy functionally graded composite material prepared in Example 1 of the present disclosure under microscope at 1500× magnification.

As shown in FIG. 2 to FIG. 7, from the surface to the core, the hard alloy functionally graded material prepared in Example 1 has compact structure and gradient transitional components and hard particle size, but no defect such as pore, bubble, crack and so on.

Reference is made to FIG. 8, which is the structural representation of the WC—Co hard alloy functionally graded composite material powder product prepared by the method in example 1 in this disclosure.

Example 2

Preparation of WC—Co Hard Alloy Functionally Graded Composite Material

The first alloy raw material was prepared by mixing WC particles and Co powder together, wherein the mass percentage of WC particles is 94% and Co powder is 6%. The additive for the surface layer mixture was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 18%, 57%, 23%, and 2%, respectively. The additive and the first alloy raw material was mixed at a ratio of 43.5:56.5 and subjected to ball-milling treatment to give a mixture for the surface layer with a Fisher particle size of 0.5 μm to 1.2 μm.

The second alloy raw material was prepared by mixing WC particles, TaC particles and Co powder together, wherein the mass percentage of WC particles is from 86.2% to 92.2%, TaC particle 0.3%, and Co powder as balance. The additive for the mixture for intermediate layer was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 18% to 20%, 55% to 57%, 22% to 23%, and 2% to 3%, respectively. The additive and the second alloy raw material were mixed at a ratio of 41.5:58.5 and subjected to ball-milling treatment to give four groups of mixture with a Fisher particle size of 0.5 μm to 5 μm for the intermediate layer.

The third alloy raw material was prepared by mixing WC particles and Co powder together, wherein the mass percentage of WC particles is 84% and Co powder 16%. The additive for the inner layer mixture was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 20%, 55%, 22%, and 3%, respectively. The additive and the third alloy raw material were mixed together at a ratio of 40:60 and subjected to ball-milling treatment to give a mixture for the inner layer with a Fisher particle size of 5 μm to 9 μm.

According to the outline structure of the component part, the expansion coefficient of surface layer mixture, inner layer mixture and intermediate transition layer mixture, an outer layer die with high-expansion coefficient, four intermediate transition layer dies and an inner layer die with low-expansion coefficient, were designed.

The mixture for the surface layer, the mixtures for the intermediate layers, and the mixture for the inner layer were put into the six composite dies in order and subjected to warm compaction molding process at 140° C. and 5 MPa to give a blank.

Under the protection of argon gas, the blank was heated to 200° C. and insulated for 1 h; then it was reheated to 250° C. and insulated for 3 h; further, it was reheated to 450° C. and insulated for 2.5 h; finally, it was cooled by nature cooling to room temperature to remove the additive in the blank completely.

The blank after the thermal degreasing treatment was put into vacuum pressure furnace and pressure sintering was carried out at 1400° C. with the protection of nitrogen gas to give a hard alloy functionally graded material after insulation cooling.

The process control test shows that the blank after removing the additives has a gradient decreasing distribution of porosity from the surface layer to the core. After sintering, the sintering shrinkage of the six blanks with different particle sizes and components from the surface to the core ranges from 1.236 to 1.241, which indicates that during the sintering treatment, the axial sintering shrinkage and radial sintering shrinkage of surface layer, core (inner layer), and intermediate transition region (intermediate layer) are almost the same, with a fluctuation lower than or equal to 0.5%.

The composition distribution of the hard alloy functionally graded material prepared in Example 2 was investigated, and the results are shown in Table 3. Table 3 shows the composition distribution of the WC—Co hard alloy functionally graded composite material prepared in Example 2.

TABLE 3 the element distribution of the WC-Co hard alloy functionally graded composite material prepared in Example 2 Distance Mass Ratio Mass Ratio to Surface of WC of Co Layer Layer (mm) (%) (%) Surface Layer(a)   0-0.4 94 6 Intermediate G1 0.4-0.7 92.2 7.5 Layer G2 0.7-0.9 90.2 9.5 G3 0.9-1.1 88.2 11.5 G4 1.1-1.4 86.2 13.5 Inner Layer (b) 1.4-core 84 16

The hardness distributions of the hard alloy functionally graded material prepared in Example 2 was investigated, and the results are shown in Table 4. Table 4 shows the hardness distribution along thickness direction of the molding part made from the WC—Co hard alloy functionally graded composite material prepared in Example 2.

TABLE 4 the hardness distribution along the thickness direction of the formed WC—Co hard alloy functionally graded composite material prepared in Example 2 Distance to Surface Layer (mm) 0.2 0.5 0.8 1.0 1.2 1.8 HV 1510 1403 1357 1321 1263 1189 Converted 91.4 90.2 89.7 89.2 88.7 87.9 into HRA

The properties of the hard alloy functionally graded material prepared in Example 2 were investigated. The results show that, the metal-based functionally graded composite material prepared in Example 2 has a density of 14.52 g/cm³, transverse rupture strength of 3280 N/mm², coercivity of 9.45 kA/m and magnetic cobalt percentage (Com %) of 9.22. The results demonstrate that the hard alloy functionally graded material prepared by the method of the present disclosure has excellent performances. From the surface to the core, the hard alloy functionally graded material prepared in Example 2 has compact structure and gradient transitional components and hard particle size, but no defect such as pore, bubble, crack and so on.

Example 3

Preparation of WC—Co Hard Alloy Functionally Graded Composite Material

The first alloy raw material was prepared by mixing WC particles and Co powder together, wherein the mass percentage of WC particles is 96% and Co powder is 4%. The additive for the surface layer mixture was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 18%, 57%, 23%, and 2%, respectively. The additive and the first alloy raw material was mixed at a ratio of 44.5:55.5 and subjected to ball-milling treatment to give a mixture for the surface layer with a Fisher particle size of 0.5 μm to 1.2 μm.

The second alloy raw material was prepared by mixing WC particles, TaC particles and Co powder together, wherein the mass percentage of WC particles is from 83.7% to 94.7%, TaC particle 0.3%, and Co powder as balance. The additive for the mixture for intermediate layer was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 18% to 20%, 55% to 57%, 22% to 23%, and 2% to 3%, respectively. The additive and the second alloy raw material were mixed at a ratio of 42:58 and subjected to ball-milling treatment to give four groups of mixture with a Fisher particle size of 0.5 μm to 5 μm for the intermediate layer.

The third alloy raw material was prepared by mixing WC particles and Co powder together, wherein the mass percentage of WC particles is 86% and Co powder 14%. The additive for the inner layer mixture was prepared by mixing polyethylene, paraffin, polyethylene glycol and stearic acid together, wherein the mass percentages are 20%, 55%, 22%, and 3%, respectively. The additive and the third alloy raw material were mixed together at a ratio of 40:60 and subjected to ball-milling treatment to give a mixture for the inner layer with a Fisher particle size of 5 μm to 9 μm.

According to the outline structure of the component part, the expansion coefficient of surface layer mixture, inner layer mixture and intermediate transition layer mixture, an outer layer die with high-expansion coefficient, four intermediate transition layer dies and an inner layer die with low-expansion coefficient, were designed.

The mixture for the surface layer, the mixtures for the intermediate layers, and the mixture for the inner layer were put into the six composite dies in order and subjected to warm compaction molding process at 130° C. and 6 MPa to give a blank.

The blank was immersed in gasoline at 65° C. for 60 h to partially remove the additive, and then under the protection of argon gas, the blank was heated to 200° C. and insulated for 1 h; then it was reheated to 250° C. and insulated for 3 h; further, it was reheated to 450° C. and insulated for 2.5 h; finally, it was cooled by nature cooling to room temperature to remove the additive in the blank completely.

The blank after the thermal degreasing treatment was put into vacuum pressure furnace and pressure sintering was carried out at 1400° C. with the protection of nitrogen gas to give a hard alloy functionally graded material after insulation cooling.

The process control test shows that the blank after removing the additives has a gradient decreasing distribution of porosity from the surface layer to the core. After sintering, the sintering shrinkage of the six blanks with different particle sizes and components from the surface to the core ranges from 1.238 to 1.242, which indicates that during the sintering treatment, the axial sintering shrinkage and radial sintering shrinkage of surface layer, core (inner layer), and intermediate transition region (intermediate layer) are almost the same, with a fluctuation lower than or equal to 0.5%.

The composition distribution of the hard alloy functionally graded material prepared in Example 3 was investigated, and the results are shown in Table 5. Table 5 shows the composition distribution of the WC—Co hard alloy functionally graded composite material prepared in Example 3.

TABLE 5 the element distribution of the WC-Co hard alloy functionally graded composite material prepared in Example 3 Distance Mass Ratio Mass Ratio to Surface of WC of Co Layer Layer (mm) (%) (%) Surface Layer(a)   0-0.4 96 4 Intermediate G1 0.4-0.7 93.2 6.5 Layer G2 0.7-0.9 91.2 8.5 G3 0.9-1.1 89.2 10.5 G4 1.1-1.4 87.2 12.5 Inner Layer(b) 1.4-core 86 14

The hardness distributions of the hard alloy functionally graded material prepared in Example 3 was investigated, and the results are shown in Table 6. Table 6 shows the hardness distribution along thickness direction of the molding part made from the WC—Co hard alloy functionally graded composite material prepared in Example 3.

TABLE 6 the hardness distribution along the thickness direction of the formed WC—Co hard alloy functionally graded composite material prepared in Example 3 Distance to Surface Layer (mm) 0.2 0.5 0.8 1.0 1.2 1.8 HV 1587 1448 1392 1320 1293 1219 Converted 91.7 90.5 89.9 89.2 88.9 88.2 into HRA

The properties of the hard alloy functionally graded material prepared in Example 3 were investigated. The results show that, the metal-based functionally graded composite material prepared in Example 3 has a density of 14.47 g/cm³, transverse rupture strength of 3240 N/mm², coercivity of 9.37 kA/m and magnetic cobalt percentage (Com %) of 9.29. The results demonstrate that the hard alloy functionally graded material prepared by the method of the present disclosure has excellent performances. From the surface to the core, the hard alloy functionally graded material prepared in Example 3 has compact structure and gradient transitional components and hard particle size, but no defect such as pore, bubble, crack and so on.

The hard alloy functionally graded material molding method of the present disclosure is illustrated clearly from the above. Examples are used to illustrate the principles and embodiments of the disclosure. The example of the present invention provided is to help people understanding the method and core concept of the present disclosure, including the best mode, so one of ordinary skill in the art can practice the present disclosure, for example, making and using the equipment or system, and combining with any of other methods in practice. It should be noted that, to those of ordinary skill in the art, improvements and modifications can be made without departing from the principles of the present disclosure, and such improvements and modifications all fall in the protection extent of the claims of the present disclosure. The scope of the present disclosure is defined by the claims and it also includes other embodiments that can be contemplated by the skilled person in the art. Other embodiments which have equivalent structural elements that are not substantially different from the literal representation of the claims, are to be included within the scope of the claims. 

What is claimed is:
 1. A molding method for a hard alloy functionally graded material, comprising: A) mixing an additive with an alloy raw material to obtain a mixture; B) placing the mixture obtained from the above step into a composite die set for composite pressure molding to obtain a blank; the composite die set comprises an outer layer die with a high expansion coefficient, an intermediate transition layer die set, and an inner layer die with a low expansion coefficient; and C) sintering the blank to obtain the hard alloy functionally graded material.
 2. The molding method according to claim 1, wherein the mass percentage of the alloy raw material in the mixture is 50% to 85% and the mass percentage of the additive in the mixture is 15% to 50%.
 3. The molding method according to claim 1, wherein step A) comprises: mixing the additive and a first alloy raw material to obtain a mixture for surface layer; mixing the additive and a second alloy raw material to obtain a mixture for intermediate layer; and mixing the additive and a third alloy raw material to obtain a mixture for inner layer.
 4. The molding method according to claim 3, wherein the Fisher particle size of the mixture for surface layer is less than or equal to 3 μm, the Fisher particle size of the mixture for intermediate layer is 0.5 μm to 5 μm, and the Fisher particle size of the mixture for the inner layer is 3 μm to 30 μm.
 5. The molding method according to claim 3, wherein the alloy raw material comprises a hard phase and a soft phase; the hard phase is tungsten carbide and the soft phase is selected from cobalt, iron and nickel.
 6. The molding method according to claim 5, wherein the mass percentage of the hard phase in the first alloy raw material is 93% to 97%, the mass percentage of the hard phase in the second alloy raw material is 84% to 95%, and the mass percentage of the hard phase in the third alloy raw material is 75% to 90%.
 7. The molding method according to claim 1, wherein the composite pressure molding is selected from warm compaction, injection molding and hot isostatic pressing molding, or a combination thereof.
 8. The molding method according to claim 1, wherein the additive is selected from the group comprising of polyethylene, paraffin, polyethylene glycol, polypropylene, polystyrene, stearic acid, dimethyl phthalic acid, dibutyl phthalic acid and EVA, or a mixture thereof.
 9. The molding method according to claim 1, wherein degreasing treatment is performed before the sintering of the blank, and the degreasing treatment is thermal degreasing and/or solvent degreasing.
 10. The molding method according to claim 1, wherein the sintering is vacuum pressure sintering or hot isostatic pressing sintering. 